1. Field of the Invention
The present invention relates to a transmission line, a resonator, a filter, a duplexer, and a communication apparatus used for radio communication and for transmitting and receiving electromagnetic waves in, for example, microwave bands and millimeter-wave bands.
2. Description of the Related Art
Ordinary RF circuits employ planar circuits that use transmission lines, such as microstrip lines, which can be easily produced and that are suitable for being miniaturized and made thin.
In the microstrip line, however, current concentration due to surface effects occurs on a conductor surface. Particularly, it is apparent at the edges, causing a power loss in a narrow region in a range of several micrometers (xcexcm) to several tens of micrometers (xcexcm) around the edges, accounting for 50% of the entire power loss. This phenomenon, called an edge effect, is attributed to the cross-sectional shape of the conductor (electrode). In planar circuits in which electrodes, such as microstrip lines, are formed on a substrate, edges always exist. Therefore, the problem of power loss due to the edge effect always occurs and is known to be unavoidable.
In this connection, RF transmission lines for aiming to reduce the current concentration at the conductor edges were suggested as disclosed in (1) Japanese Unexamined Patent Application Publication No. 8-321706 and (2) Japanese Unexamined Patent Application Publication No. 10-13112.
In each of the above publications, a plurality of linear conductors is formed at a constant pitch, parallel to a signal-propagation direction. It can be said that, in the above-described conventional transmission lines, the conductor is divided parallel to the signal-propagation direction to reduce the current concentration at the edges. However, to form the conductors with the correct line width in these structures requires very severe manufacturing accuracy, on the same order of magnitude as the skin depth. In addition, the conductor Q value is improved only within a small range of 10 to 20% of the conventional Q value. Further, depending on the dividing method, there are cases where the conductor Q value decreases, so that it is actually lower than the Q value of a single-line conductor.
Thus, in the structure in which the direction of a current path is the same as the signal-propagation direction, even when the linewidth is divided to be as thin as possible, the left and right edges still exist. Therefore, the structures are not effective enough as a fundamental solution to the edge effect problem.
In view of the above, an object of the present invention is to provide a transmission line, a resonator, a filter, a duplexer, and a communication apparatus that efficiently minimize power losses due to edge effects, thereby having superior loss-reduction characteristics.
In order to achieve the above object, a transmission line of an embodiment of the present invention is configured of at least one continuous line and a plurality of thin lines each branching from the continuous line and having a predetermined length.
According to this structure, other thin lines having the same shape may be arranged adjacent to the one thin line. In this case, since physical edges exist when microscopically viewed, a weak edge effect occurs at the edge of each of the thin lines. However, when the plurality of the lines are macroscopically viewed as a whole, the edge on the left side of one of the connected thin lines exists adjacent to, for example, the right edge of another one of the connected thin lines. Therefore, substantial edges in the line width direction do not exist; that is, the edge is not noticeable. This allows the current concentration at the edges of the lines to be efficiently reduced, thereby minimizing the entire power loss.
The thin lines do not deteriorate the transmission characteristics of the transmission line since the successive thin lines together function as a single high-frequency transmission line. Because respective ends of the thin lines are connected to a common continuous line, current transmitted in the continuous line flows into the respective transmission lines. Therefore, magnetic fields are induced around the respective thin lines. Due to the coupling between the magnetic fields, the thin lines are electromagnetically coupled with each other. As a result, a high frequency signal can be propagated via the successive thin lines along the extension direction of the continuous line.
Also, in the transmission line of the present invention, the branching direction of each of the thin lines may be slanted with respect to the continuous line. In this case, the direction in which the thin lines extend has a component extending in the signal-propagation direction for all of the lines, thereby allowing the edge effect to be efficiently minimized.
Also, in the transmission line of the present invention, the aforementioned individual thin lines may be connected, and the aforementioned continuous line connects identical portions of the aforementioned lines. For example, each of the thin lines is arranged to have substantially an integer multiple length of half the wavelength corresponding to the transmission frequency, and central portions of the individual thin lines are connected. With this arrangement, both ends of each of the thin lines become open ends, and portions that represent nodes in the voltage amplitudes are connected via the continuous line. Alternatively, by connecting both ends of each of the thin lines via the continuous line, both ends of each of the thin lines become short-circuited ends, and portions that represent antinodes in the current amplitudes are connected.
According to these structures, electromagnetic-field distributions (voltage and current distributions) on the individual thin lines are forced by the continuous line to be uniform. This increases the efficiency of the reduction of the edge effect due to the close arrangement of the individual thin lines.
Also, in the transmission line of the present invention, the thin lines may be curved lines, and a controlled capacitive coupling or mutual dielectric coupling between each pair of thin lines may be arranged.
Also, in the transmission line of the present invention, a line width of each of the thin lines may be not more than the skin depth of a conductor of each of the lines. By this structure, currents that flow to maintain magnetic fields that pass through gaps between left sides and right sides of the individual lines are spaced apart by suitable distances so as to cause interference at the left sides and the right sides thereof. This minimizes reactive currents deviating in phase, thereby allowing the power loss to be significantly reduced.
Also, in the transmission line of the present invention, each of the thin lines may be a thin-film multilayered electrode, having overlaid thin-film dielectric layers and thin-film conductor layers. By this, the skin effect in the direction from the substrate surface to the outside of the electrode can be reduced. This allows a further reduction in the power loss to be obtained.
Also, in the transmission line of the present invention, a dielectric material may be filled in each gap between the adjacent thin lines. By this, short-circuiting between lines is prevented, including when the lines comprise thin-film multilayered electrodes as described above.
Also, in the transmission line of the present invention, at least one of the individual lines of the aforementioned thin lines may be configured using a superconductor. In this case, the low-loss characteristics of the superconductor advantageously allow a high Q value to be obtained at a level lower than a critical current density.
A resonator according to an embodiment of the present invention is configured using the aforementioned transmission line as a resonant line. This allows a resonator having a high unloaded Q value to be obtained.
Also, a filter of an embodiment of the present invention is configured by providing a signal input/output section in the aforementioned resonator. This provides a filter that is small and produces a small amount of insertion loss.
In addition, a duplexer of an embodiment of the present invention is configured using the aforementioned filter as at least one of a transmitting filter and a receiving filter. By this, a small duplexer that produces a small amount of insertion loss and that is small can be obtained.
Furthermore, a communication apparatus of an embodiment of the present invention is configured using at least one of the aforementioned filters and duplexer. In such a communication apparatus insertion losses in RF transmitter and receiver portions can be reduced, and also, quality in communication with respect to, for example, transmission speeds, can be improved.